This invention relates to apparatus for generating an exchange pulse train for use in a telecommunications system, in particular in a time division multiplex ("t.d.m.") digital exchange system.
Supplying telecommunications systems, in particular t.d.m. digital exchange systems, and the transmission lines of such telecommunications systems, with an exchange pulse train whose pulse frequency is determined preferably servo-synchronously entails problems, two of which are ensuring the accuracy and breakdown resistance of such a pulse supply system.
In this respect it is known from German Auslegeschrift No. 26 36 150 that a phase-accurate synchronisation of fundamental generators can be achieved in carrier frequency transmission systems with a standard frequency transmitted over long distance traffic networks by means of digital, phase-accurate frequency adjustment for course setting, and analog frequency adjustment for fine setting. For the coarse adjustment a digital phase regulating loop is formed which, in respect of each beat cycle between the standard frequency and the reference frequency of the oscillator which is to be pulled into step, produces only one digital adjustment step for the purpose of frequency correction. In addition, an analog phase regulating loop is provided whose pull-into-step adjustment range corresponds to approximately double the adjustment step of the digital regulating loop, and the center of the analog adjustment range lies midway between consecutive response points, assigned to a beat cycle, of the digital regulating loop.
In the event that the analog adjustment range is exceeded, an adjustment step is triggered by the digital regulating loop which causes the phase of the reference frequency of the oscillator which is to be pulled into step to return to approximately the center of the adjustment range of the analog phase regulating loop, and the adjustment value formed in this way is stored.
With a frequency adjustment method of this kind, in the event of the breakdown of supply of the standard frequency, the stored adjustment value is retained (see also Tekade Techn. Mitt. (1978), pages 24 to 28) so that interruptions in the standard frequency transmission do not affect the short-term frequency stability of the oscillator; however, in the event of a breakdown of the frequency adjustment device, frequency stability is no longer assured.
In addition, German Offenlegungsschrift No. 27 37 713 discloses a t.d.m. digital exchange sytem comprising switching network devices which are provided in duplicate and during disturbance-free operation operate in parallel to one another and each of which assumes approximately half the traffic. It is already known to provide in duplicate an exchange pulse generating arrangement which supplies the exchange and the lines outgoing therefrom with an exchange pulse train whose bit frequency is preferably determined by an auto-synchronous and/or a servo-synchronous process, in which case a first of the two exchange pulse generating arrangements supplies one of the duplicated switching network devices and the lines outgoing from the exchange with an exchange pulse train which has been produced, independently of the second exchange pulse generating arrangement, and the latter supplies the other of the duplicated switching network devices with an exchange pulse train which is synchronised to be in phase with the exchange pulse train supplied by the first exchange pulse generating arrangement. The exchange pulse trains produced in the two exchange pulse generating arrangements are virtually equal in phase, so that when the bit streams conveyed via the two halves of the switching network are combined at the output end, only negligible phase differences occur between the two bit streams.
Also in the event of a mutual switch-over of the roles of the exchange pulse generating arrangements, as the result of which previously leading and led exchange pulse generating arrangements are now led and leading, respectively, only a limited phase jump occurs which does not impair the synchronism of line regenerators inserted into the lines outgoing from the exchange.
At the same time the possibility is provided that, in the event of a disturbance or intervention for purposes of servicing, there is no need for undesirable phase jumps since, having been rerouted, the two halves of the switching network can be supplied from only a single one of the exchange pulse generating arrangements for which purpose, in a further development, the individual switching network devices can be supplied with the exchange pulse trains of the two exchange pulse generating arrangements from an appropriate switch-over device.
In this known system, when servo-synchronisation is used the master pulse train is fed only to one of the two exchange pulse generators. If, for reasons of resistance to breakdown, it is desired to increase the circuit redundancy and, in the case of duplicated exchange pulse generating arrangements to frequency-synchronise both exchange pulse generators using different master pulse train sources in a servo-synchronous process, it is necessary to limit deviations from the desired phase quality of the two exchange pulse trains to a maximum permissible value even when the master pulse trains (having a fundamentally identical pulse train frequencies) supplied to the two exchange pulse generators can differ in phase by up to 2.pi..